Well, I'm not sure exactly which parts you are interested in buying. If you are talking about current technology, it pretty much varies between each type of component.
There is one thing that I would suggest to you though. If you are in the process of "saving" money to buy these parts to buy a PC, "don't" buy them little by little until you eventually have everything you need.
Instead, save the money that you are budgeting while tracting each week the cost of the components you are planning on buying. When you have all of you budgeted money saved and are ready to purchase all of your parts, you will have saved at least some money on certain parts from the price decreases over time.
Another way to do it is, if you already have a PC, you can buy certain parts that you will use in the new PC, and temp. use them in the current PC. Like "Sound Cards", "Optical Drives", "Additional Storage Drives", "Cases"..
I would certainly wait until the last possible minute to purchase a Motherboard, CPU, RAM, Video Cards and Power Supplies. These are the components that always seem to be changing and very dependent on each others performance.
it depends on what hardware you are looking at, and how high-end you are going
computer parts are always decreasing in value, they are a depreciating asset. the longer you have one, the less it will be worth. the lowest price you'll see on new hardware will be just before it is phased out.